1. Field of the Invention
The present invention relates to a technique for activating a fuel cell in a low-temperature atmosphere.
2. Description of the Related Art
A typical example fuel cell has a membrane electrode assembly in which an anode and a cathode are provided on either side of a solid polymer electrolyte membrane. The membrane electrode assembly is placed between a pair of separators so as to support the membrane electrode assembly. In such a fuel cell, a fuel gas (e.g., hydrogen) is supplied to a power generation plane of the anode, while an oxidizing gas (e.g., air including oxygen) is supplied to a power generation plane of the cathode, so as to produce a chemical reaction. The electrons generated in the chemical reaction flow to an external circuit, providing DC (direct current) electrical energy. An oxidizing gas such as oxygen or air is supplied to the cathode, and the hydrogen gas, electrons, and oxygen gas react at the cathode, thereby generating water. Therefore, the fuel cell has less effect on the surrounding environment and thus has become the focus of attention as a driving source for vehicles.
Generally, the operating temperature of this kind of fuel cell is approximately 70 to 80° C. However, at low temperatures, the power generating efficiency is degraded; thus, it is difficult to obtain satisfactory starting performance at a low temperature. Accordingly, when such a fuel cell employed in a vehicle is activated in a low outer temperature (e.g., below the freezing point), the starting operation requires considerable time.
In order to solve this problem, Published Japanese Translation No. 2000-512068, of PCT International Publication No. WO97/48142, discloses a system in which electric power is supplied to an external load of a fuel cell so as to promote the reaction and increase the temperature due to self heating, thereby improving the starting performance.
On the other hand, U.S. Pat. No. 6,103,410 discloses a system in which a portion of hydrogen which functions as a reaction gas is mixed with air so as to produce a reaction by using a catalyst at the cathode and to generate heat of combustion, thereby improving the starting performance.
However, in the former method of using the self heating, if the temperature of the fuel cell to be started is below the freezing point, considerable time is necessary for self-heating the entire fuel cell which has a large heat capacity. In the latter method of burning a part of the hydrogen, extra hydrogen used in the starting operation is necessary in addition to hydrogen consumed in power generation; thus, a larger tank for storing hydrogen is necessary and the space for arranging peripheral components is limited.
In addition, the heat obtained by the self heating of the fuel cell may be insufficient for warming up the fuel cell at the start of the operation, and water generated in the fuel cell during the warming up may freeze in the fuel cell.